1. Field of the Invention
This invention relates to a permanent magnet magnetic circuit to be used for a semiconductor manufacturing apparatus or a similar apparatus requiring a unidirectional magnetic field (having linear lines of magnetic force in a desired region) and, more particularly, it relates to a permanent magnet magnetic circuit that is especially useful when restrictions are imposed on its attachment or arrangement as well as to a magnetron plasma processing apparatus incorporating such a permanent magnet magnetic circuit.
2. Description of the Related Art
There is known a conventional permanent magnet magnetic circuit designed to generate a unidirectional magnetic field and comprising a U-shaped yoke having a pair of permanent magnets attached to the inside surfaces of the respective legs of the yoke in such a manner that the N pole of one magnet is opposed to the S pole of the other magnet. In such a permanent magnet magnetic circuit, only a unidirectional magnetic field directed from the N pole of said one magnet to the S pole of said other magnet is formed and contains least of unnecessary perpendicular magnetic field components.
In a permanent magnet magnetic circuit as described above, however, the region where the unidirectional magnetic field is generated is surrounded at three sides thereof by the magnetic circuit. Therefore, the magnetic circuit is not useful where restrictions are imposed on its attachment or location.
There is also known a plate shaped permanent magnet having N and S poles disposed at opposite edges thereof. A permanent magnet magnetic circuit of this type is a most fundamental one and can generate a region of an approximately unidirectional magnetic field above or below a major surface of the plate-shaped permanent magnet. The magnetic circuit of this permanent magnet is free from restrictions of attachment of a magnetic circuit unlike a permanent magnet magnetic circuit having a yoke as described above.
However, although a permanent magnet magnetic circuit of the above mentioned Second type produces a (horizontal) magnetic field in parallel with the plate-shaped permanent magnet at the central area thereof, unnecessary vertical components drastically increase with the distance from the center of the plate-shaped permanent magnet, making the magnetic field far from ideal.